[refactor] Migrate from Mocha+Chai to Jest (#6079)

* [refactor] Migrate from Mocha+Chai to Jest

This change migrates all the existing unit tests
- to Jest's global expect and matchers from chai's imported expect, asserts and matchers.
- to Jest's describe/test from mocha's describe/it

The majority of the mechanical changes to tests are achieved through running jest-codemods. The only two note-worthy manual tweaks:
1. Setting a testURL of http://localhost in jest config and adjusting a few tests to leverage this value instead of relying on about:blank.
2. Re-enabling ExploreChartPanel_spec which was previously commented out as we cannot have empty tests with nothing in it with Jest. :)

This change also removes dependencies to Mocha and Chai.

* Remove the test:one command as it now does the same thing as test.

* Fixing lint errors. The diff looks large but is large done through `yarn run lint --fix`

The only noteworthy change is the one in eslintrc for tests. The env has been updated from mocha to jest.

* Adding eslint-plugin-jest and further modify tests.

- One small fix in sqllab's Timer Spec for a test that is not using the spy it created for testing.
- Deletion of a duplicated test caught by eslint-plugin-jest.

* - Make istanbul coverage work with Jest.

- Remove dependency on stand-alone istanbul and babel-istanbul as they're built-into jest. Yes!

* Attempt to fix dynamic imports in tests.

* run sequentially and log heap usage

* - tweaking maxworkers for travis and specifying coverageDirectory for codecov

- remove dynamic import in shim.js now that it is set in babelrc for tests only.

superset/assets/spec/javascripts/messageToasts/utils/getToastsFromPyFlashMessages_spec.js

@@ -1,5 +1,3 @@
-import { expect } from 'chai';
-
 import {
   DANGER_TOAST,
   INFO_TOAST,
@@ -11,14 +9,14 @@ import getToastsFromPyFlashMessages from '../../../../src/messageToasts/utils/ge
 describe('getToastsFromPyFlashMessages', () => {
   it('should return an info toast', () => {
     const toast = getToastsFromPyFlashMessages([['info', 'info test']])[0];
-    expect(toast).to.deep.include({ toastType: INFO_TOAST, text: 'info test' });
+    expect(toast).toMatchObject({ toastType: INFO_TOAST, text: 'info test' });
   });
 
   it('should return a success toast', () => {
     const toast = getToastsFromPyFlashMessages([
       ['success', 'success test'],
     ])[0];
-    expect(toast).to.deep.include({
+    expect(toast).toMatchObject({
       toastType: SUCCESS_TOAST,
       text: 'success test',
     });
@@ -26,7 +24,7 @@ describe('getToastsFromPyFlashMessages', () => {
 
   it('should return a danger toast', () => {
     const toast = getToastsFromPyFlashMessages([['danger', 'danger test']])[0];
-    expect(toast).to.deep.include({
+    expect(toast).toMatchObject({
       toastType: DANGER_TOAST,
       text: 'danger test',
     });